a rojas



Filed March 23 lil' F. A, ROJAS May 15, 1923. 1,455,735

FA A. ROJAS RHEOSTAT Filed March 23 19?? 4 SheetsfSheet 2 N VEN TOR B Y 0W, f iiD/ww a ATTORNEYS May 15, 1923. 1,455,735

F4 A. ROJAS RHBOSTAT Filed March 23, 1922 4 SheetsSheet 5 I N VEN TO By w May l5, 1923.

F. A. ROJAS RHBOSTAT Flnd March 2.3. 1922 4 Sheets-Sheet 4 E c r www@ 5141x211 foz 351g Hoz c OM Patented May 15, 1923.

UNITED STATES PATENT OFFICE.

FLORICEL A.. ROJAS, OF NEW YORK, N. Y.

RHEOSTAT.

To all 'L Ulmm it may concern.'

Be it known that I, FLORICEL A. ROJAS, a citizen of the Dominican Republic, residing at New York city, in the county of Bronx,r State of Nerv York,l have invented certainnev and useful Improvements in Rheostats; and I do hereby' declare the followin'g to be a full. clear, and exact description'of'the invention, such ,as will enable othersV skilled in the art to which it appertains to make and use the Same.

This .invention relates to" 'Iheostats of the compres'sibleresistance type. The resistance element of such4 vrheostat's consists essentially f amass of particlesof conducting material' making imperfect contact with one another and usually held within a mass of non-conducting material. The conductivity7 of such amass as a whole is a function of the surface contact 'of the particles, and as this is increased and decreased, the current flow variesaccordingly. The number of particles is so great and the Contact so numerous that practicallyT infinitesitual current variations can be effected with .pressure changes. Ordinarily such a "resistance, consists' of a fibrous nonconductinggi,r material such as asbestos combined With a powdered conducting material such as graphite and held between two relativelyv movable members, (one of which is usually fixed), the Amovement of which, by compressing and expanding the material, increases and decreases its conductivity accordingly. Sometimes the asbestos is dispensed with and the conductive elements used in granular form without the addition of a non-conducting element, or this latter can itself be granular. In order to ,get the best results the resistance material should be secured to both ot the relatively movable members, so that a positive expansion, as Well as compression. may be effected, and this is true Whether the non-conducting elcment of the resistor be'a resilient, fibrous material. such as asbestos, or Whether the resistor be granular.

The resistance element and its embracing` members or container are. mounted `in sonic sort of.frame or support and means provided for moving: the members to Vary the resistance of the compressible and expansible material. Thus the rheostat may be said to consist of two essential parts, the support, and the resistor itself, including its terminal plates and resistance material. The present invention has for its object improvements in both of these parts of the rheostat.

Among,r the chief diiiiculties encountered in the use ot rheostats of this type. are those which result from the internal heat generated in accordance with Joules law. This h eat is not onlilikely to burn out the resistance, but is even more likely to interfere with the steadiness of adjustment of the instrument. The evolution of this heat is inevitable, but the rheostat can be so constructed to render its effects largely innocuous, and this is one of the objects of the present invention. In riew of this ,varying heat loss it is obvious that Where steadness of adjustment is desired, it is first of all essential that the rheostat as a whole have a substantially zero temperature coefficient of resistance. Two Ifactors determine the temperature coeflicient of the rheostat; first. the temperature coetlicient of the conductii'e element of the resistance, and second, the resultant coefficient of thermal eX- pansion of therheostat as a Whole, including the resistance material itself. The temperature coefficient of tht` conductive elements can be made to approach zero by the judicious mixing of: substances havingI positive and negative coefficients, such, for example,

carbon und metals. To secure a mixture the resistance of which is absolutely unaffected by temperature changes, is, of course. impracticable. As this ideal is unattainable, error must be made on the side of safety and the mixture have a slightly positive coefficient; that is. if metals are used as the positire constituent. they must be present in more than the theoretical quantity. But, the rate at which the conductivit;7 of metals changes with temperature is much less than the rate of change for carbon. 'and Jfor this reason. even in a mixture of carbon and metals having a zero eoeflieient, the metal constituent must largelj.v predominate. This is objectionable. first. because large amounts of metals tend to short-circuit the rheostat, and second, because no easil)v oxidizable metals should be used. and the cost of those relatively nonoxidizable metals. such as copper, silver, gold platinum. etc.. is deterriug if not prohibitive. As stated in my pending application, Serial No. 516,134, filed November 1S. 1921, conductive silicon being ine pensive, non -ozidiable Y at ordinary temperatures, and possessing a high positive temperature coefficient, is admirably adapted for this use. Accordingly this invention contemplates a resistancematerial the 'conductive elements of which consist of a mixture of graphite, or other carbon forms, and conductive silicon.

It is not always necessary that the conductiie elemeutsbe used in the form of a homogeneous composition or intermixture of particles in order to obtain a zero temperature coefficient of resistance. The different conductiye elements may-sometimesbe made up intoseparateresistance units of proper relative. and used inseres. Under certain ciricpms'tances, ,when using silicon. and graphite; have found this particularly adtgntageousfor, reasons which will ,he pointed Qn in .detail Laterff l 'ut .azeretemgerature coeliicient in the conductive elements themselifes is not enough tcl-'give a zeroeqetlicienttothe rheostat as a whole. The generated heat causes the resistance material, both conducting and noncondugting constituents, to expand, which increases the points or surfaces of Contact among the conducting yparticles and thus increasesthe conductivity of the material. At the same time the metallic parts ofthe rheostat are expanded tending to counteract the :elect of-.the'internal expansion of the resistance material. In order to control the-Serenella@ factors I .hare provided a delicate thermostat in the form of a metallic disc supported at its edges, and upon which the container. for the resistance material rests. As this disc is heated it expands and relievesV the pressure upon the resistor, thus compensating for the internal expansion of the particles. I have also provided means whereby the effective expansion of this disc can be nicely regulated so that for any given rheostnt and resistor the resultant temperature coellicient can be brought to substantially zero. lVhen, however, it is not necessary that absolute steadiness of adjustment heattained` l can dispense with the thermostat and secure :in approximate accuracy b v means of thc composition alone. Thus. if :l given rhcostat has. as a whole a negativo temperaturecoellicicnt. that is, if the increase in conductivity due to the exs pansion of and increased contact among the conductive particles of the material itself more than counterhalances the decrease in ronductivitr due to the expansion of the me tallic parts of the rheostat as a Whole. the resistance can he.. giren a positive coellicicnt. to compensate for this under average work ing conditions. 'this may be done by inf creasing the proportion of silicon. A ther inostat can, of course. he used in comhina/ tion with such a resistor.

ln compoundingl n resistance :nai crial haringiiny predetermined temperature cnc-,llicient, when the conductive elements are enibeddcd in a non-conductive clenient, the thermal expansion of the latter must bc reckoned with 'to some extent. rl`hus a gran ular mixture of silicon and graphite, without asbestos designed for :i zero temperature coefficient vwould boot somewhat different proportions from a similar mixture intended to impregnate asbestos. The ditlerencesarc, however, not great and the proper proportions can be readilyA determined by any one skilled in the art and having this specificationhefore him.V Although asbestos wool is the material most commonlyemployed as the nonfconductire element of the resistor, it sometimes becomes necessary: to subjectvlhe rheostat to temperatures above those at which asbestoscan be successfully-employed. UnderV those circumstances powdered granular or Haked substances, such as silica, beryl or flake mica can be used.l These materials possess the requisite refractoriness, high electrical -re. sistance and thermal conductivity. Their cotlicients of thermal expansion are practically soclose to that of asbestos that they can be substituted substantially neig-lit for weight for asbestos when compounding resistance materials have n predetermined temperature Ecoefficient of resistance. l

A substantially zero temperature coetli-4 cient ln a compressiblc resistance rheostat as a whole is not only essential for accurate work. but it -is also essential that thercsistance material itself vharen zero or .slightly positive Veoefiicient to prevent local overheating and possible burning out of the material, It is impossible to distribute the conductingl material evenly throughout the mass and it is impossible to apply pressure evenly over 4the entire body of the resistor. ll-'ith material haring a hip-h negative cooflicicnt, such :is carbon. its conductivity in creasingr with its temperature. it is clon r that :i concentration ol' thc innlcrial or :in unA crenli' distrilnitcd pressure resnltingfin :in increase in local current flou' would give rise to au ine\ila.:lc incicnsc in temperature nilli increased cnri'cnt flou'. thc vicious c vcle thus sot up possibly rcsiiliing in lnirnin;r ont the` resistor. Still further to j fuur'dallainst this lccnl mcrhsntin;r l place within the Inntcrinl :i pair or pairs of nictallic plntcsot properly chosen niclals or ninnc thc movable compression platos thcnlsclrcs of proper metals` which pairs of plates :1ct :is thermocouplcs, and. when the current is flowing' in the propcr direction. tend to equalize the temperature hv zillsorhing heut at their hol cnils and generating it :it their cold. in accordance with Peltiers law.

These limei-nosed plates also perform other functions. They serre to conduct heat from llfl design I ioth support and resistor. The rcsistance material is put up in closed con l-ainers or cartridges provided with means forattaehment to the supportreadily positioned and just as easily removed. Or tho remorablecartridges can be done away with and the resistance material itself furnished in' such form that it may be readily slipped Ainto place in a. permanentcontainer an properly secured in place.

vVarious other improvements are contemplated by this invention, such as the method ot preparing the resistance material for use1 the method of packaging this material and inserting it in the rheostat, means for lining the container to prevent adherence of the ginpliite-etheseand others are 'shown in the 'accom anfing drawings and will be de- 'scribe more in detail in connection there with,

In thesedra-wings Fig. 1 is a vertical sectional 'view of a rheostatlconstructed in accordance with the present invention,l the longitudinal dimensions being somewhat exaggerated for the purpose of more clearly showing' the strncture. Fig. 2 is a section along lineY 22 of Fig. 1, showing the Ine-ans whereby the resistor is removed from the 3 and L 'are modified 'forrnsrof the resistor itself held within its container or cartridge., These views also show'the inter-p gsell serve the various functions noted above. Fig. 5 is a plan vieny `and Fig.V 6 a .vertical section througha still further modified type of rev silstor in which the in 4osed plates are psrtieula-rly designed as the h o-couples. Fig. 'I is. a vertical section through another type oflresisto'r in which thq resistance element is in the forni of't-hree superposed slabs secured to'one another by a. suitable adhesive hut without .intervening plates Fig. 8 is a 'voiticalsection through a stillfurther modied' 'forln'of resistor. 9 and 10 are plan' and side views respectively of a resistance unit. `Fig 11 is a vertical section through a still further moditied'form of re sistor in which the-cartridge'or Vcontainer is permanently, fixed to, the frame of the rhcostat. and into which the resistance material is' designed to be introducedby the user. This figure also discloses a. modified form4 of thermostat. Fig. 12 is a vertical section through a. carton containing a resistor com prisiug three supcrposcd units. these units being designed for insertion within a rontaincr suc-h as shown in Fig. ll by thc user of the instrument; Fig. 13 is a vertical section through another type of resistor'showing a different form of interposed ,plate and diti'erent'means for securing the resistor von tainer to .the'frame of theY rheostat. Figs. 14 and l5 are vertical sections through anothe'r'type of resistor in which springs are employed to cause the material to expand taken and compress in approximate accordance with elastic laws, so that the potential drop will be substantially uniform throughout the thickness of the material; Fig- 16 is a vertical section throu means for positively expanding and compreing the material in stages, as will be ex plained ingreater detail latergand Fig. 17 is a vertical section through a resistor in which units of different conductive elements are placed in series.

ln describing .the present invention in connection with the drawings I shall first describe Fig- 1, whichnshows in detail asupport for a -resistor and means forz expanding and compressing it for compensating for thermal expansions. This support or frame of .the rheostat will be described in connection with one type of resistor and the'method of mounting theresistor within' the support and rexnoyin'g it will b e clearly set forth.4 -This havingfbeen done the description will then proceed to a consideration of the various types of resistor which form the greater part of the present invention and which may be employed in the support shown in Fig, 1 or conceivably 'in other supports.

The rheostat shown in Fig. 1 comprises a housing mounted upon a"suitable' base'and designed to hold within it thegvarious parts of the rheostat. Thishousing comprises 'a lower portion 1 `and an upper oiition hinged together at 3 and provi ed with locking means 4. When it becomes necessary to change the resistor these two parts of 1 the housind are swung apart and the ezichange, made as will be described later. As shown, the resistor is mounted withinthe lower part of the'easing land the upper part 2 carries the mechanism by means of which the expansion and compression of the resistor is effected.

I shall now describe one type of resistor and its mounting within the casing. The resistor 5 as shown in Fig. 1' comprises a container or cartridge 6 for the compressiblc resistance material.V 7, this 'cartridge comprising upper andA lower metallic plates 8 and 9 respectively and side walls 10 of fle;- ihle material, preferabl)r asbestos cloth. Plates 8 and 9 are lil-ml)r secured to the side walls 10. one type of joint beingr shown in the drawing at 12 Thus the resistance niaterial is herrueticallj;r sealed within the container and cannot spill out. The resistance material is positively secured to plates 8 and .l by means of a suitable adhesive. or otherwise, as disclosed in my Patents Nos. 1,315,- iifl and 1,366.945, so that. relative mo\'c ment of the plates effects a positive expansion as well as compression of the material 7.

Plates S and 9 are provided with outwardly projecting screws 13 and i4 respectively gh a. resistor provided with r and thermostatic means )orted b means of a rin;r 17 suoocrted u on a series of lugs 1S projecting from the inner walls of the housing 1. Both disc 1G and ring 17 are insulated from the lugs 18 by suitable insulation 19 and held in place by screws 20.

I shall now describe the mechanism whereby the upper plate 8 is moved relatively t lower plate 9 to compress and expand the resistance4 material. Extending through the upper faceof part 2 of the casingand journaled therein is the shaft of a hand wheel 22, the lower end of which makes threaded engagement with a nut 23 to which is connected, through the intermediary of 'a universal joint 24, a flanged casting 25 suitably insulated'from the shaft ofthe hand wheel and the casing by means of -insulation .26. Vertical movement of this casting 25 is effected through turning of vthe hand wheel 22 but stability is attained through the use of. a leaf spring 2T which is 'likewise secured' to 'the casting 25 and extends upwardly on either side and is held fast upon bosses 28 extending inwardly from thewalls of the housing. This leaf spring is suitably insulated from the housing by means of insulation 29. A cupshapedcasting 21 is secured through screw 13'to' -the platev 8 and makes electrical contact'therewith. This casting extends upwardly within the flanges oi the casting 25, as clearly shown shown in Fig. 1. Casting 25 is prorided'with a number of set screws 27 which extend inwardly through its de pending ilangev and enter an annular groove 30iwithm the upper outer face of casting 21, asishownlbest' in Fig. 2. Access to the set screwsis had'through apertures 31 in the housing 1.

iFroin the foregoing description it will be clear 'that movement of plate 8 toward and away from plate 9 is accomplished through the 'turning of hand wheel 22, compression of the material 7 being permitted because o'z -the flexibility of the absestos cloth side walls 10 of the cartridge 5. Upward movement of plate 8 effects, a. positive expansion of material 7 due t0 the adherence of that material to the plates 8 and 9, and this upward movement may be limited by means of suitable stops, not shown. The method of conducting the current through the resistor. the operation of theshunt which is thrown in parallel with the resistor when the conA ductivity of the latter has reached I'uil current capacity, and the operation and control of the thermostat for compensating for thermal expansion and thus attaining a zero temperature coefficient yet remain to be described.

The housing is provided with two terminals or binding posts 32 and 33, both of which are properly insulated from the casing, the first at 29 and the second at-34. The path ot the current is as follows: From terminal 32 through a bolt 35, leaf. spring 2, casing 25, set screws 27, casting 21, plate S, resistance material 7, plate 9, base cast ing 15, metallic disc 16, a metallic strip 36, a bolt 38 and terminal 33.

As is common in rheostats of this type, a` shunt is provided, in parallel with the resistor, which is thrown in when the conductivity of the latter approaches full current capacity, thus relieiing thefresistor when no good could be accomplished by passing the current throughit. The shunt constructed in accordance with the present invention comprises a rod 39 extending up` wardly from the periphery of disol and making electrical contact therewith,'and a set screw 40 mounted within a lug 42 on the flange of casting 25. As the'casting25 isV lowered with the turning of hand Wheel 22 the lower end of set'screw v40 ,eventually makes contact with rod 39 andshuntsthef current around the 'resistor'. The time at which this shunt is thrown in can be regulated by means of the set screw, access to which is had through an aperturep43 in the upper face of the housing.v In order that there be no 'sudden shock when the set screw 40 impacts upon rod 39, the latter is provided with a spring cushioned cap '41', which yields when contact is made.4 i i As stated above, theE discv 16 acts not' 'only to support the resistor, but serves as a thermostat as well. The heat generated in :iccordance with Joules law causes a' thermal! expansion of this disc, which in turn 'relieves the pressure upon the resistor and tends to reduce the surface contact among the conducting particles, and hencel increase the resistance, thus compensating`for the decreased resistance due to the expansion of these particles. As it is a physical impossibility to design a. thermostat which will automatically and without adjustment com pensate for the resistance changes in any given resistor, under any conditions of use, it is imperative that adjusting means be provided. ln the present invention these means consst of a spherical disc 43 supported upon a threaded bolt 44 mounted within :i bracket l5 secured to :i bracket 45 depending from the ring 17. Access t0 the lic-.id of bolt 44 is had through an aperture 46 near the base of the housing, and the disc 43 caused to approach or recede from the disc i6. Thus by moving member 43 up till wardly the effective expansion of disc 16 is reduced, whereas if member 43 is downwardly outof all contact with disc 1G a. condition permitting maximum expansion is realized.

The operation of this device as a rheostut will be clear from the foregoing, but it may he woll to review briefly the manner of removing an old or burned out resistor and insertingr new one. .-t screw drivel' is inserted through apertures 31 andset screws 27 with; drawn from contact `with casting 21. Lock L is then untastened and the upperpart of the housing 2, swungr backwurdly about its hinge; Casting 21 1s then unseren-ed from the resistor, the resistor itself removed from its buse 15 and a new resistor substituted.

'lhe reassembly of the part is sim'p'ly and" rapidly made and after readjustment of the thermostat and the shunt' the device: is in readiness for use.

Before proceeding'to a description of the various forms of resistors and' a discussion of the resistance' material ,itself I 'shrill call attention to Fig. 11, which disloses 'a modified form of thermostat.' This thermostat consists n'ot only of the met'zilli' disc ltbut has superposd thereon between that. dist' and the container'ortlie'resistfarlcefa Second' and compound thermostat comprising two m'etiils having different' eotlicients'hf efpan- Slnll. upper disc 47, which is Scur'd'lrirctly t'o the base of the containei,'andf a louisiand dished dise 4S, the peripl'eral'edgesjof tle.

the disc 4S. The' disc 47 sion thandisc 48, the' discs Vheilig' 't'deg'lfor' and bra's's respectively.'

example, of Viron vLinder the inlu'euce or heat th' 'disc 48 vill oxp'and more Ythan disc Tand cause' a col. lapse of the compound member, thu's releasing the pressure exerted upon thelresistince. T his composite thermostatls usefuI 1n cases where great the'rniostntlc action l's a high positive temperature coefficient.

Coming noir to the constilction o f there'- sistors themselves, let us first consider the form shown in Figs. 3, 4, 5, 6, 7, 8, llgmd ll. one shown in Fig. 11, are, sealed, that is, the

resistance material is held within a'contiineiwhich is entirely enclosed', andwhich is intended tohe put up iii final form by the In the or'm shown it c'mprises''ati dsi'red in order to correct the eil'ect of 1:es1stor s having 4.

All of these, with lthe exception of the itself, This casting is provided with the necessary depressions to receive the ends of set Screws 27. The resistor shown in F ig. lll differs from the other resistors in that instead of being provided with the projecting' screws 13 and 14 for securing it in place within' the rheostat support, it is provided with threaded holes() designed to cooper ate with outstanding screws 6l ou the rhcostat support. A resistor of this typ'e possesses alight advantage over the other forms in that it is easier lto pack and ship.

1t will also be noted that the resistauc'e's shown in all of these resistors are.l in cfo'ntradistinction to that shown in the resistor mounted in Fig. Linade up in a plurality or' layers. TheseV layers, as shown ii1` Fig'sl 6., 7, 8, 11 and 13ar'e entirely 'separate and distinct from each other, whereas in' 3 and t'there is' no such sharplde'tiiitio'i'i and they are merely4 st-ip'arat'd by plates 5 1 ihifl'h are embe'd'de'd within and secured to'the mijtrial. These whicl''the'y eifeptad'also the division ot fibres' 'f theslis'toswool are not long enoughto"riichcoripletely across the thickness of A"the Aordinary resistor, i from movable electrddeftfsttionary electrode, and,l the resultis'Jthiat thersistance is likely to develop Weak spots `Within' the material so that i'pon the r'pnsion thehres pull aiviayfromieacl'i other and form gas. This is highly un-r desirable, and in localized overheating and probame out, of the material. Where', however, these interposed plates- 5l aref. used some of the fibres actually spanthe bietweenthem, and between th'em and theV end plates, jso that the probability of is gxatly reduced. vIn orderthus to strengthen the resistance almost any material. can "be employedjtov form these plates, providing, of cours'e, it is an electrical, conductor. Thus metals vcan be used, as shoiv'n most simply in 3, 4 and 8, Vor carbon,- as shown in Fig. 13. Or, as a matter of fact, the plates can bel eliminated entirely Aand the' several layers or units or resistance-material caused to adhere directly to each by means of intermediate; layer of conducting adhesive. as .shown in Fig. 7;

eV secondA function which these' plates perform is that of conducting the internally generated heat from the interior of the resistance to its surface, and to do this it is, of course, necessary to use go'od thermal conductors, chief among which are themetals. To perform the third possible'funrtion these plates must also-he madeof metal and of particular metals too. This third n plates, well as plates sheen* inrias, simu iapefqi-in i;

materiiilapart to form' air gapsfunction is the special redistribution of heat in accordance with Peltiers law. Thus, if two metals are employed which bear the proper thermo-electric relation to each other, they form a thermo-couple Within the mass, and should localized overheating occur near one of their junctions they will tend to absorb this heat and liberate it at their other or cold junction. In order to obtain this result it is, of course, necessary that the current pass in u predetermined direction. Thus, in the case of Fig. 3, if the current is passing through the resistor from top to bottom and the upper plate 51 is zinc and the lower one copper, these plates will tend to equalize the heat in accordance With Peltiers law. This etfert is not the major purpose of the plates, but is without doubt beneicial and is one of the factors which should be taken into account.

The resistor shown in Fig 6 is especially designed to utilize this thermo-electric effect. Here the upper electrode 52 is made of zinc and the lower electrode 53 made of copper. Each intermediate plate 51 is in itself a thermo-couple composed of upper plates 54 of copper and lower plates 55 or' zinc, separated along the major portion of their length and by means of suitable insulation 56. If due touneven pressure or other cause, joints at the right hand end be heated above those la-t'the left hand end, a thermo-electric current will be generated in the direction indicated by the arrowheads, and will tend to cool the right hand end of the resistor and heatl the left hand, thus tending to equalize the temperature. In this case it is assumed that the current is Howing from the electrode 52't0', electrode 53. This same effect can, of course, be obtained without the use of intermediate plates where the movable end plates themselves are made of roperly different metals. i

Ye resistor shown in Fig. 1 'and described in connection with that ligure, is made up of rigid lend plates andxflexible sides'. This construction is also'sliown in Fig. 6, but Figs. 8 and 13 disclose containers of an entirely different form in which the side walls are rigid and the ends flexible. To the inner face of these flexible asbestos cloth diaphrafrms 57, conducting discs 58 are secured. @In Fig. 7 the lower face is rigid, but otherwise the structure of the l container is similar to that shown in Fig. 8.

lVhen graphite is used as the constituent of the conducting element of the resistance, it has been 'noted that it tends to form a coating over the surface of the container walls, which coating being electrically conductive acts to short circuit the cur rent around the resistance and causes trouble. Now l have discovered that if the container walls are made of glass coated with vitreous enamel :is disclosed in my copendin(y application, Ser. No. 516,134, or provited with a protective sheet of mica, this difliculty is overcome, and the graphite does not form a coating. Thus, the resistor shown in Fig. 6 is provided with a cylindriral sheet of mica 59, and the resistors shown in Figs. 7, 8, 11 and 13, as well also as those shown in Figs. 14, 15 and 16, are provided with vitrcously enameled side walls 60.

I shall now take up a description of the resistors shown in Figs. 14, 15 and 16 which are designed to solve the problem of ex panding the material so as to obtain a uniform otential drop throughout its thickness. ach of the containers shown in these figures is provided with vitreously enameled side walls 60 and lexibleends 57 to` the inner faces or which are secured carbon 58. The resistance material is the' or inary asbestos wool combined with yconducting powders and provided at intervals with interposed carbon plates 51. The problem is t effect a uniform compression and expansion.

I shall first consider Fig. 14. Each of the plates 51 is provided with ah'ub ofiiisulating material 62 provided with flangesf, designed to telescope with corresponding flanges on an adjacent hub and'withcorresponding flanges projecting inwardly from the end plates 58. Held within these flanges and bearing against the faces of the hubs themselves are three coil springs 64. Let us suppose that the lower end of the resistor is held stationary and pressure applied through upper plate 58. The severalI sections of spring 64 act as one entire whole and transmit this pressure equally7 toi the material. Thus, the material in lthe uppermost zone is compressed no more than that in the lowermost. Uponrelease of the pressure and an upward-movement of the lat- 58, a uniform expansion is similarly e ected. This form of resistor is shown provided,

with an internal shunt 70.

The resistor shown in Fig. accomplishes this same result by means of c. slightly different mechanism. Here the side Walls are provided with a series of annular shoulders 65 upon which rest annular' rings 66 having at intervals upstanding spring ears 6T which support plates 51.

The pressure beingr substantially uniformly applied throughout `the mass of resistance material employed in the resistors of Figs. 14 and 15, it is necessary only that the composition of this material be uniform throughout in order to insure a uniform potential dren. When using the form ot' resistor shown in Fig. 16, this is not the case. as will be evident from the followino'.

Extending downwardly from upper plate 58 (Fig. 16) is a projecting rod 68 which passes through central apertures in the lates l will' non' be piate'sSi ahxdi'is insulated therefrom. This ficiation, the composition of the 'conductingr ro'd' is provided with' a series of rings 69 element of the resistance should most spaced from the discs in a manner which described, Let u'sassuineagain that the lower plate is stationary and that a pressure is applied to the upper one. As this u per plate 58 starts on its downward rbfte ent itcompresses the material in thel upper zone. 4The material in the other' zones casier-caress is also compressed to a slight extent, but due tothdfaet that the material as :i whole does not det 'as L trile; ela'stic body, this` compres sin is' relaltifely's'light' and most of 4the ae'tion'foeeurs iii the one immediatelj'adjacent 'th rri'ii'ing j part. Ordin'aiily, this waguttconnne; uiep'pperl mangimi being c tipieis's'glfto' "a'nhichgreater exteiit thnthe 1"ei"." Hbw'exeirwhen" theA upper 'zidne linas rate i and the second Azone is `being cmx pahtledlbegiiining with the Alowest and continiiifflip'iifariily in succession. Although a resistopbuiltas shown in 16 equ'aithe" tssiiie in'the several' 4zones, it is, ne'terth e's"s"'n'ssary to tary the; composirial and, as the lower dises'hnrenot vet come intoplay as effecting ther expansion of the material, the uppermost zone hns been exA panded morethan the lowermost. Thus. to havel a uniforin composition and potential drop at this point it is necessary that when in the iriitial unexpanded condition the upperinb'st zone hare a composition richer in the conductingY elements than the lower-most.

The composition of the intermediate zum: will lie in between these tuo extremis,

As stated m the cari); part of this spericases have a st 'xbstantiall'y zero, or slightly positiveitemperatdre coefficient. For practical reasons the resistance material as :i'ivhol'c is' compounded to have this coetiicieiit. Thus', the 'ronductive 'elements' :are combined withthe' asbestos or other noneondrlctiv'e element a'nd the teinp'era'tur eoe'iiicient of the wh'o'le fied fou given working coiiilitions, thus tali'ng 'account the QtherinalA expansion of thmaslbesto's. 'As examples of composit'i'ons liviitg substantially zero temperature coefficients of resistance I instance th fb'l'lowin'g: i

, Parts hy w weight.

Silicon. 8 Graphite Asbestos 4 Silicon 9 Graphite, 2 Asbestos Z Silic'om 9 Lamp black 1 sbes'tos 2 iVer the 4asb'estos removed the ratio of silioon to carbon would be slightly varied.

It is to bel noted that thev ratio of s'il'icoito carbon in thse: mintures high, fowvtho resistance of 'si'iicon'je\ih'e1 rhated isA high, so thht' the specific resistance of tliese coinpounds is "high. I have discovered that in' intimate mix'turesof silie'on'and graphite, thesilicon Ueedresheated by conduction and radiation from thencarbon. A its resistance that it fails to carry its proportionate share ofthe current andifor'` this reason it bec'oj'm" necessary greatly toiincrease pro ortio'ns relative tothe earbon. And so t e result is 4a resistor of high specific resistance. In orderv toovercom'e this diiculty I ma biiiid the resistance n'aterial as shown in g. 17, the graphite and asbestos indicated at 75 being,r separated from the silicon and asbestos at 7 B, the tw'o being preferabl f though not necessarily1 separated by ameta plate 77. In this way t e two constituents being in series the current mustl pass through each and the` silicon add its effect t0 Vthat of the graphite. YVhen alrranged in this manner the ratio of silicon to graphite instead of being about four to one, as in the examples given, is about one to four, as shown in Fig. 17.

ln making up the layers or units of resistance material employed in these resistors, l t-.ilezisbestos wool fibres, coat them with the conductive powders, as for example, in a tumblingvf drum, and then when they are coated, felt them and cut from the felt. units of the proper dimenfions, Were lamp tinck is employed as thc carbon constituenti make a suspension of this lump black in alcohol and saturate'the asbestos wool with it. Experiment has shown that the asbestos is impregnated by the lamp black in this manner. VThe same method can be employed with graphite but with less success. In both instances tumbling in a drum may be combined with the saturation step.

The resistors shownvin Figs. 1,3, -l, 6, 7, 8, 13, 14,15, 16 and 17 are designed to be made up bythe manufacturer and shipped to the use-intheorm shown, but I have also pro- 'vided ineens whereby fa permanent container or cartridge maybe'employed and the resistanoe material itself inserted by the user of the-theostatfivsuch a. containeris shown in Fig.'11...Thelmovableplate 71is secured diirectlyf,.to the' resistance material on the one iside' and=tothe`leaf spring 27 and the shaft ofhand wheel-22 on the other.A Should the Vfes'istanceburn out or should it become necessary: to .change or renew it,.th e upper meniber'.2 of -thehousing isswung open, `thus tearinglose .the-contact between the resistance and the plate 71. The old resistance can then be removed andanyadhering fibres scraped from Athe container andthe plate 7 l. Tbej new resistance is then placed within the container. The mannerof doing this will nowsbe described.

. E;The' resistance materia-l, .either in a single .piece orin properly designed-umts, is put up -withinacarton72 shown in Fig. 12, this cartonbeingprovided witha` removable bottom 73 and a4 top-74 designed-to be pushed with- :inthe carton.- Inorder to insert the new resistance within the container shown inFig. 11,1it is only necessary to remove the bottom 73, place the' carton over the container, bottom down, and press upon the top 74.-. This .will eject the resistance and position it within the container. Before this is done, of course, the bottom of the container and the late 7l should be coated with a suitable adliesive. This being done the upper part 2 of the housing is swung into close position,` the plate 71 forced down upon the resistance and held in that position until the adhesive has time to set.

The adhesive used can be any suitable adhesive which is conducting, such as glue iinpregnated with powdered carbon, but Where the resistance material is to be glued to :i metallic plate, I have found it advantageous to. mix with the glue a saltof a metal so related to the metal of the plate in the electro-motive force series thatthe metal of the salt' will be replaced and deposited upon the late, thus making an admirable Contact.. lior example, where the late is of'iron, I mix copper sulfate with t e glue.

As discussed in connection with Fig. 1G, the composition of the resistance material is determined by first determining the compositiou which it must have for minimum rcsistanre. .This beingl determined, the range o!" [iisd resistance is then determined by its vxpzeiision. vWhen in a mean position its resistance should be equal to that of theexieri-:nl resistant-e in the line, (the heat loss in the resistor being greatest here), so atthis point the potential drop should be uniform, :ind therefore the distribution of the conducting material should be uniform. Where the expansion itself is not equal throughout ille thickness of the resistor, then those, parts r-:hich are expanded most must be richer in conductive material, und this in direct proportion to their relative expansions..

`ii'herc the temperatures to which the rlicostat is to be submitted are above the working temperatures of asbestos, other nonconducting material should be used. Grannlnr silicon` bei-y1 and the like lend themselves quite satisfactoril Y for this purpose, but I have found that flalied mica is superior lo these, for the reason that it ossesses aV larger resiliency of its own. l en compounds of this nature are used, it is necessary to increase the number of units in a multiple icsistor'to such a quantity as will insure safe distribution of the potential drop throughout its thickness. Thisprocedure will be essential inasmuch as, due to the absence of the fibres of asbestos, apowder or granular compound will not have the high coeiicient of elongation, which is 4characterl istie of asbestos wool compounds, and it will bc necessary to rely upon the much ,smaller resiliency of such a compound assisted, how.- over, by the positive drawing aprt`of, its mass by adherence of the outer particlesto the plates. As stated above, thesematerials can be substituted for the asbestos substantially weight for weight.

The principles underlying the construction and operation of a practical and successful rheostat of the compressible resistance type have been set forth at some length in the foregoing spelication, and the application of these principles has been made clear b r references to preferred mechanisms and methods. It is clear` however` that'the principles themselves are broader than 'these specific illustrations, and it is believed that many a man skilled in the art, after studying this disclosurev could construct other pieces of apparatus, compound other mixtures and devise other methods which, although differing from the ones specifically given here, would yet embody the principles herein disclosed for the first time.

This application is acontinuation in part of my copending application Ser. No. 510,13. filed November 18, 1921.

I claim:

1. A coinpressible resistance material having;Ir a substantially zero temperature coeflirient, comprising carbon and conductive siliconA .i :'onijii'essiblc rr-sstance material huring a substantially zero tampa-rature cecilicient, comprising a mixture of `graphite :mil conductive silicon. the ratio of graphite to silicon being about one to four.

Il. A compressible resistor having a suhstantially zero temperature coefficient '.romprising a unit ir. which carbon is the conductive element in series with i unit in which conductive silicon is the conductive element. Y

Li. .1\ compressible resistance material havv ingIr :i substantially zero temperature coeliiv cient comprising asbestos impregnated with :i mixture of conductive silicon and graphite.

5. "A comprcssihlc resistance material havin.;r a substantially zero temperature coeflif'ient comprisingr asbestos impregnated with .1 mixture of conductive siicnn and graphite. the proportions by weight being'snbstantialiy asbestos two parts ;'conductive silicon four parts and graphite one part.

6. A compressible resistance coriiprising,r Atwo or more separate layers of compressible resistance material joined h v means of a conducting adhesive. i'

In a compressible resistance rheostat, the combination of a compressible resistance comprising two or more separate layers of compressible resistance material separated by :nid secured to interposed plates of conducting material, and means for positively expanding and compressi'ng'the resistance.

8. Irina compressible resistance rheostat, the'combination of a cinpressible resistance comprising two or more separate layers of compressible resistance material separated by and secured to interposed metallic plates. and means for positively expanding and compressing?r the resistance.

E). A resistor comprising two relatively movable metallic plates, a oompressible resistance material between th plates, the plates bein;r of diterent metals so related ltoleach other Ithat when the current flows through the resistor in a predetermined direction, the plates act thermo-electrically, in accordance with Peltiers law. and re distribute the heat in the resistor.

1U. A rompressible resistance provided interiorly with a thermo-couple= which. when the current is Howing'through the resistance in a predetermined direction, will absorb heat at its hotter end and generate it at its colder end. in conformity with Peltiers law. i

11. A compressible resistance unit, comprising a layer of felted, resistance material made np of a non-conducting material, impregnated with conducting material 12 compressible resistance unit, comprising a layer of asbestos fibres, which Fibres are coated with a conducting powder.

I3. The method of making compressible lcsistnncc units. which comprises coatingr nowronilurting fihi-ons material with conconducting plate. "i

ducting powder` felting the fibres thus coated and blanking ont the units to the desired size and shape.

14. The method of making compressible resistance material which comprises saturating asbestos libres with n suspension of conduetive particles.

15. A rheostat comprising an expansible and compressible resistance material. and means for expanding-and compressing said material, the resistance of the material being substantially uniform throughout its thickness when approximately in its mean position.

16. The combination of a compressible resistance rheostat comprising two'relatively movable electrodes, including between them a eompressible resistance material7 and an external resistance in 'serieswith it, the composition of the resistance material being 'such that the potentialdrop between the electrodes will" be substantially uniform through the'tliiekness' o'f the' `material vwhen the internal resistance of the rheosta't equals the external 17. The combinationof a compessible resistance 4rlieostaf coni rising' two relatively movable elctiodesfihc'lding'.between them a oompressible resistance materiali and an external resistanceY i"se`ries'1vith it, the composition of 'the resistajne'cmterial besuch' that the potential'- drop'between the electrodes afi-IIv be' Substantially miam throughl the thiclislsio'the material `when the Heat generated within' the ress'tancis a.

` 18. A c'ompressible resistor, comprising a 'sealed coiia:ner2 -compssible resistance withinthe'iioxtamer'fand means for rernovably montingtl' es'istot'vithir the' frame o`fa rli'eositaf:"i7 'f l 'i 19.' vAtti''co'x'press le resistor,A comprising a tened warmer having s' fie'xipiswall. a conductiig;V ',frll'iien red'to the inner' face of said; v'vall; 'nd c'oxiip'ressible resistance within th'' conlaihfand adhering, to? `said 20. A compressible resistorfcomprising a. compressible' rsistance, means for positively expandir@ and coiiipxcmig the resistance. and resilient 'eas-whiehdistritiifefthe pressure throughout xthe''tlliclrn ess ofthe' rgistaner.=. 2L ,A wmpwihle meer, qqmprising a compressible r'sstiiznce, an interposed-plate within the resistance, two relatively movable compressing .arid epanding' en plates. springs integr-@seed 'bange 'uirnd entes and the interposed ,'plate; so that the'etfect of the relative I'io'vin'to'f the nd'plate's'is transmitted thrli'gh thethicknssiof the res istance. i

22. A compressible resistor, comprising a compressible resistance, an interposed plate within the material, two relatively movable compressing and expanding end plates. means moving with the movement of the end plates to move the interposed plate after the end plates have moved a predetermined distance.

23. A compressible resistor, comprising a stationary end plate, a movable plate, compressible resistance material included between said plates, an intermediate plate within the material and substantially parallel with the other plates, a projection extending inwardly from the movable plate, means on said projection for engaging the intermediate plate andimparting a delayed movement to it.

24. The method of operating a compressible resistance rheostat, which comprises acting upon separate layers of the resistance material in succession to expand or compress them.

25. A oompressible resistance material, comprising carbon, a comminuted refractory, thermally conductive, electrically resistant material. and conductive silicon.

26. A compressible resistance material comprising graphite, fiaked mica and conductive silicon.

27. A eompressible resistance material comprising the following ingredients in substantially the following proportions: graphite one part, laked mica two parts and conductive silicon four parts.

28. A compressible resistor, comprising relatively movable end plates, a plurality of interposed plates, and comminuted resist-ance material distributed among the plates.

29. A rheostat comprising a support, a resistor, and means for removably mounting the resistor within the support.

30. A rheostat comprising a support, a compiessible resistor, and means for removably mounting the resistor within the support.

31. A rheostat comprising a support, having upper and lower members, means within the lower member for supporting a compressible resistor. and means carried by the upper member for effecting :in expansion and compression of the resistor.

32. A rheostat comprising a housing having upper and lower members. a thermostatic disc mounted within the lower member, a compressible resistor mounted upon said disc. and means carried by the upper member for effecting an'expansion and rompression of the resistor.

A rheostat comprising a housing having upper and lower relatively movable members, a compressible resistor mounted within the lower member. vertically movable means depending from the upper member` and means for connecting" said movable ine-ans lo and disconnecting it from the resistor.

34. A rheostat comprising a housing having upper and lower members, a coinpressible resistor mounted Within the lower member, a threaded shaft extending through the upper member, a movable part operatively connected to said shaft, and means for securing said movable part to the resistor.

35. A rheostat comprising a housing having upper and lower members, a compressibleA resistor mounted within the lower member, a threaded shaft extending through the uppermember, a movable part operatively connected to said shaft, a leaf spring supported by the housing and secured to the movable part to steady it, and means for securing said movable part to the resistor.

36. A rheostat comprising a housing having upper and lower members, a compressible resistor mounted within the lower member, a threaded shaft extending through the upper member, a movable part operatively connected to said shaft through a universal joint, and means for securing .said movable part to the resistor.

37. A rheostat comprising a housing having upper and lower members, a compressible resistor mounted within the lower member, a threaded shaft extending through the upper member, -a flanged casting operatively connected to said shaft and set screws passing through the casting to secure it temporarily to the resistor.

38. A rheostat comprising a housing having upper and lower members, a compressible resistor mounted within the lower member, a movable part supported by the upper member and designed to be secured to the resistor to effect an expansion and compression thereof, and a shunt, one contact carried by the resistor support and the other by said movable part, the parts being designed to contact and shunt the current around the resistor when its resistance has been reduced to a predetermined point.

39. A rheostat comprising a housing having upper and lower members, a conr pressible resistor mounted Within the lower member, a movable part supported Aby the upper member and designed to be secured to the resistor to effect an expansion and compression thereof, and a shunt, one contact carried by the resistor support and the other by said movable part, one contact of the shunt being adjustable, the parts being designed to Contact and shunt the current around the resistor when its resistance has been reduced to a predetermined point.

40. A rheostat comprising a housing having upper and lower members hinged together. a compressible resistor removably supported within the lower member, a movable part carried by the upper member, and means for operatively securing the movable part'. to the resistor to expand and compress it in relation of the parts permitting the lli.)

readily rviiiovzil nml replzwmiieiit of the ro- `sistor. I

41. ln :i compressihle resistance rheostat. :1 resistor` :i support therefor Comprisin n flexible metallic disc. which by ekp'zmdiiig nets thermostaticully to relieretlie pres's'lire upon the resistor as its temperature"rises, und regulntalole menus forgoverning 'the effective expansion of the disc,y

42. In n Compressihle resistance rheostat, a resistor, a support therefor comprising; u, pziir of discs of different metals secu'r'ed'tognther at their edges', which by differential 1,455,735 i me.:

ex )ansion act thermostaticully to relieve tli ,"'p'ressure ilpfhl the resistor as its' temperature'rise-s.

'43. i :imrheostat` a pile of resistance units, each miit hving vn'riabl'e specifici"- sistance und means positvelyiactig up'on tlrtil'e tix' in'ase r'liecrease the density onf the uil' srv'hole toch'an'ge its conductiblity.

"Tn testimony whereof l aix my signature. 't'

FLOR-IGEL A. ROJAS. 

